An antenna stabilization system is generally used when mounting an antenna on an object that is subject to pitch and roll motions, such as a ship at sea, a ground vehicle, an airplane, or a buoy, for example. It is desirable to maintain a line-of-sight between the antenna and a satellite, for example, to which it is pointed. The pointing direction of an antenna mounted on a ship at sea, for example, is subject to rotary movement of the ship caused by changes in the ship's heading, as well as to the pitch and roll motion caused by movement of the sea.
U.S. Pat. No. 4,156,241 to Mobley et al. discloses a satellite antenna mounted on a platform on a surface of a ship. The antenna is stabilized and decoupled from motion of the ship using sensors mounted on the platform. U.S. Pat. No. 5,769,020 to Shields discloses a system for stabilizing platforms on board a ship. More specifically, the antenna is carried by a platform on the deck of the ship having a plurality of sensors thereon. The sensors on the platform cooperate with a plurality of sensors in a hull of the ship to sense localized motion due to pitch, roll, and variations from flexing of the ship to make corrections to the pointing direction of the antenna.
U.S. Pat. No. 4,596,989 to Smith et al. discloses an antenna system that includes an acceleration displaceable mass to compensate for linear acceleration forces caused by motion of a ship. The system senses motion of the ship and attempts to compensate for the motion by making adjustments to the position of the antenna.
U.S. Pat. No. 6,433,736 to Timothy, et al. discloses an antenna tracking system including an attitude and heading reference system that is mounted directly to an antenna or to a base upon which the antenna is mounted. The system also includes a controller connected to the attitude heading reference system. Internal navigation data is received from the attitude heading reference system. The system searches, and detects a satellite radio frequency beacon, and the controller initiates self scan tracking to point the antenna reflector in a direction of the satellite.
An antenna stabilization system may include an azimuthal positioner, a cross-level positioner connected thereto, an elevational positioner connected to the cross-level positioner, and an antenna connected to the elevational positioner. The system may also include respective motors to move the azimuthal, cross-level, and elevational positioner so that a line-of-sight between the antenna and a satellite is maintained.
It has been found, however, that movement of one of the positioners may cause undesired movement of another positioner, i.e., the azimuthal positioner may be coupled to the cross-level positioner, or the elevational positioner. Accordingly, larger, more powerful motors have been used to compensate for the undesired motion. It has also been found, however, that the use of larger motors may cause overcompensation, and an accumulation of undesired movement, which may increase errors in the pointing direction.
A tachometer feedback configuration, including a base-mounted inertial reference sensor (BMIRS), has been used to reduce the coupling between positioners. This configuration, however, may increase pointing errors due to misalignments, phasing, scaling and structural deflections between the BMIRS and the positioners.